The technical field relates to controlling motor vehicle aerodynamic drag. The vehicle ride height of a truck, the nominal vertical distance between the chassis of a vehicle and the ground or road surface, is adjusted as a function of trailer type and load to reduce aerodynamic drag. Control over the adjustment process is implemented through automatic and manual inputs which also mediates for road conditions.
The average new vehicle fuel economy for heavy-heavy long haul trucks (25+ ton loads with engine displacement ranging from 11 liters to 15 liters) in 2003 was 6.1 mpg. The annual average improvement in fuel economy for such vehicles from 1995 to 2010 was 0.88% with about two thirds of the improvement coming from changes made to the engine and transmission, and one third of the improvement coming from better aerodynamics and reduced tire rolling resistance. Engine changes have related primarily to improvements in electronic fuel injection, combustion improvements and reduction in friction. Aerodynamic improvements have included increased use of cab fairings and spoilers. Second generation radial tires exhibit some reduction in rolling resistance over the prior generation. Continued refinement of these technologies should continue to provide some gains for a few years, but continuing to meet the historical pace of year on year improvements of 0.88% a year is unlikely.
Vehicle aerodynamic resistance is a function of the vehicle's drag coefficient, its effective frontal area and its height above the driving surface. In truck/trailer combinations the trailer usually has a greater frontal area than the truck due to extending above the truck's height. On such vehicles aerodynamic resistance can be reduced by streamlining the truck and incorporating fairings to the truck which expand the frontal area of the truck to match the trailer but which extend the streamlining of the truck to the trailer.
Extending streamlined fairings from the truck to the trailer is less effective, and even self-defeating, if the trailer is not as tall as the truck and the fairings simply operate to increase the frontal area of the truck. In such cases there is more to be gained in terms of reduced drag by lowering the truck to as close to the road surface as practical. However, dynamic application of a system for changing the vehicle ride height of heavy-heavy commercial trucks has not been considered practicable. For one thing, heavy-heavy commercial trucks have been designed to operate at a specific ride height to match a fixed fifth wheel height required by most trailers.
Air suspension systems have become common on trucks and trailers and one of their attributes has been their ability to maintain a constant vehicle ride height. Air suspension systems allow the carrying of loads of varying weight at a constant height because the air pressure in the air spring bladders which support the load can be varied as required to level the vehicle and to bring vehicle ride height back to nominal distances. Closed loop control systems have been employed to maintain vehicle ride height. The load an air suspension carries can be adjusted over a wide range, without changing the air spring height, simply by changing the air pressure. In addition to changing the load-carrying capability, a change in air pressure also affords changing the spring rate without a significant change in the natural frequency of the suspension system.